With the performance improvement of the power semiconductor switching devices, the power electric devices also developed from widely used half controlled power semiconductor switch, thyristor, to present excellent full controlled devices, so as to drive the rapid development of power electronic converter technology, and kinds of converters appeared based on pulse width modulation (PWM) control.
Voltage source converter (voltage source converter VSC) technology is a type new converter technology based on turn-off device IGBT (insulated gate bipolar transistor) and pulse width modulation (PWM) technology. Because IGBT is able to turn-off itself, the flexible HVDC system can work in the passive inverter mode, no need of additional commutation voltage, and the end can be a passive network; VSC can simultaneously and independently control the active power and wattless power, it is flexible and convenient to control; VSC did not need wattless power supply at the AC side, but also play the role of STATCOM to dynamically compensate AC bus wattless power and stable AC voltage; there is no communication need between VSC station, and easy to constitute a multi-terminal system.
Conventional HVDC thyristor valve has been widely used in power system. It has obvious advantages at the high voltage long distance transmission and the interconnection of large area power grid. Owing to the switch device turn-off control limits, the valve thyristor is prone to commutation failure, it cannot be used for weak AC system interconnection. And all the voltage source converters based on full-controlled-devices are able to self turn-off without additional commutation voltage, and the end can be a passive network; they have active power and wattless power fast independent control ability and wattless power dynamic compensation ability, can provide reactive power and voltage support for the system on the emergency situations; there is no communication need and it can realize the unattended, that is contribute to the construction of smart grid.
At present, the transmission projects in operation based on the voltage source converter are all adopted two or three level converter structure, but this structure has some problems:    (1) two level voltage source converter adopts pulse width modulation technology, the output voltage of the converter is high frequency jump, which brings about a series of problems like high dv/dt stress of equipment, high system loss, high noise and hardly electromagnetic environment. Even if the three level voltage source converter adopted, it just can alleviate the problem to some extent only.    (2) two level and three level voltage source converter uses switches series technique, because of the switching devices parameters dispersion, each switching device needs to design a complex static voltage balancing circuit, a dynamic voltage balancing circuit and a snubbed circuit, that causes potential problems to the valve during operation. In addition, the switching devices series technique require high performance switches, that needs very high pressing process, it is difficult to manufacture parts, and it is easy to come out assembly defects.    (3) the output voltage waveform of two level and three level voltage source converter has high distortion rate, high harmonic content, so it needs to design the filter circuit to reduce the adverse effects caused by harmonics on the system.
In these figures:
1. slide rail; 2. capacitor; 3. bypass switch; 4. switch to protect thyristor valve; 5. IGBT control board box; 6. motherboard; 7. power pack; 8. main control box; 9. thyristor valve and switch control board box; 10. water pipe; 11. IGBT control board box; 13. IGBT control board; 14. radiator; 15. IGBT; 16. IGBT bursting diaphragm; 17. waterproof cover board;